This invention relates generally to an improved method of making useful hollow bodies of thermoplastic material, and more particularly relates to making hollow bodies from copolyester polymer elastomer material, and the products resulting therefrom.
As discussed in U.S. Pat. No. 4,198,037, issued on Apr. 15, 1980 to the assignee of the present application, elastomers have been widely used in the past for various products, including springs. One of the more recent useful thermoplastic elastomers is a copolyester polymer elastomer such as sold by E. I. duPont de Nemours & Co. of Wilmington, Del., under the trademark HYTREL. As explained in said patent, HYTREL is made from three ingredients, namely, diemethyl terephthalate; polyglycols such as polytetramethylene ether glycol, polyethylene ether glycol or polypropylene ether glycol; and short chain diols like butanediol and ethylene glycol. Generally, this product can be used to form thermoplastic elastomeric products. Similar elastomers may be produced and sold by others.
Generally, this type of polymer elastomer material has inherent physical properties that make it unsuitable for use as a compression spring. However, the recently issued patent referred to above describes a method by which the copolyester polymer elastomer material can be treated for rendering the material usable as a compression spring. Generally, that treatment, to convert the elastomer into compression spring material, comprises the application of a compressive force to a body of material which compresses the body in an axial direction to an extent greater than 30% of its previous axial length, measured in the direction of the applied pressure.
In continuing with the experimentation and development of the compression spring material disclosed in said patent, it has been found in accordance with the present invention that a physical modification to the elastomer block before the application of a compressive force thereto has an appreciable and unexpected effect on the physical characteristics, function and utilization of the final elastomer product. Generally, it has been found pursuant to this invention that the provision of a central core or opening in the copolyester polymer elastomer body, before the application of an axial compressive force to the body, has substantial beneficial effects on the resultant product. The use of the resulting hollow elastomer body as a compression spring is enhanced by changing the spring characteristics of the body and enlarging the scope of applications where such a spring can be utilized. The invention allows the physical characteristics of the hollow body to be varied easily to accommodate the loads and deflections needed in particular spring applications. Elastomer springs with various spring rates thus can be easily produced by using this invention. The resulting hollow elastomer body also possesses characteristics which make it useful in applications other than as a compression spring.
The provision of a core opening extending at least partially through the elastomer body before the application of a compression force has been found not to cause the sidewalls of the body to collapse, as may be expected. Rather, the sidewalls of the body and the core opening expand outwardly in a transverse direction as a result of the application of the compressive force to the body. The resulting sidewalls are generally uniform in thickness and symmetrical about the axial center-line of the core opening and the core opening has been greatly enlarged to define an essentially hollow body from the elastomer material. The presence of the core thereby changes the physical characteristics of the elastomer body.
The functional characteristics of the elastomer body are also changed by compressing the material with a core opening extending at least partially therethrough. When the body is utilized as a compression spring, the spring characteristics of the hollow body have been changed, compared to a solid body of the same material. The spring rate is changed, and the amount of dynamic and static energy which can be stored by the spring has been varied. The functional characteristics of the hollow elastomer bodies produced pursuant to this invention thus expands the flexibility of design and the scope of application for spring units utilizing copolyester polymer elastomer materials.
Moreover, the operating characteristics of the hollow elastomer body produced by this invention can be varied in a simple manner by changing the shape and size of the core opening provided in the body before compression. For example, if a lighter spring with a higher spring deflection rate per unit load is desired, the size of the core opening can be enlarged to correspondingly reduce the thickness of the resulting sidewalls of the hollow body. Similarly, a stiffer spring can be produced by selecting a smaller core opening so that the increased sidewall thickness produces a stiffer spring. The shape of the elastomer body, as well as the shape of the central core opening can also be varied to suit particular applications. For example, the body can be cylindrical, oval, generally rectangular or square in configuration. The core opening likewise can be circular, oval, rectangular or square in cross-section. In the preferred embodiment the transverse shape of the core opening and the body are similar and co-axial, so that the symmetry of the body is maintained.
In addition, the hollow bodies in accordance with this invention are simple and economical to produce, compared to hollow bodies made by prior techniques. Instead of requiring expensive and cumbersome cores or internal machining to produce a hollow opening in a body, the present invention requires only the provision of a core opening in the elastomer body prior to compression. This opening easily can be drilled into or through the body with conventional drilling equipment. Alternately, if the elastomer body is to be molded, the central opening can be molded into the body prior to compression. If the initial elastomer body is to be cylindrical, for example, a simple pipe section can be used for the mold for the body as well as for the core opening.
Also, relatively minor modifications in the jigs and fixtures used to produce the hollow bodies will result in variations in the shape of the final product. For example, the jigs and fixtures used to shape the hollow body can be arranged to provide one or both axial ends of the body with a reduced collar or neck portion which could be used to mount and position the body in particular industrial applications. Similarly, the jigs and fixtures can be arranged so that the final core opening extends only partially through the body. One axial end of the body will thereby be provided with a solid end wall that is desirable for particular applications.
The hollow elastomer bodies produced in accordance with this invention also can be designed and used for purposes other than industrial compression springs. They are useful for example, as isolation and vibration dampers, such as in motor mount applications. They are also useful as energy absorption bumpers or cushions. The hollow elastomer bodies produced pursuant to the invention also have sufficient symmetry about their axial dimensions to be utilized in various applications as rollers or wheels.